Various types of aerial work apparatus are well known as excellent tools for lifting workers or material upon a platform to elevated and oftentimes obstructed locations. Such equipment is provided with an extendable boom that can be rotated about its counter-weighted base as well as elevated to varying angles off the ground. The platform is commonly provided with a work basket to hold workers and material but it may instead consist of a material-handling mechanism such as a winch or fork-lifting tines.
Each model of this line of apparatus typically has a rated capacity for the load it may carry at the distal end of the boom. This rated capacity is based on safety standards to arrive at a prescribed limit for the load to be carried whenever the boom is substantially extended and level to the ground. Exceeding this rated capacity technically risks overcapacity since any greater load is presumed capable of causing the apparatus to overturn. Determined efforts are made therefore to avoid overcapacity given the obvious potential for serious injury to workers on the apparatus as well as in its immediate vicinity should the equipment tip over.
A single capacity for the platform has long been a convenient safety parameter since keeping the lifted load under this figure was a simple way of avoiding the possibility of overcapacity regardless of boom position or boom length The drawback with this approach is that platform capacity varies not only with the load at the end of the boom but also with the boom's degree of elevation and range of extension. In particular, the size of load that can be safely lifted can exceed the rated capacity by simply not fully extending the boom. As a consequence, full advantage of such equipment is never achieved because the usefulness of the machine has been intentionally diminished.
One solution to this problem has been to provide the aerial work apparatus with charts that allow the operator to manually determine the maximum platform capacity for a certain combination of boom angle and length. These charts would diagram zones within which the prescribed platform capacity was applicable. Even though these charts would often be permanently affixed to the apparatus at a location easily seen by the operator, the charts were subject to damage or defacing. They were also overlooked or ignored by careless operators. More importantly, many operators were often completely unaware of the actual size of the load being lifted on the platform to enable them to even begin to use the charts.
Measuring or sensing the load on the platform of an aerial work apparatus in an automatic fashion would serve as a welcome alternative to placing sole reliance upon the operator to measure and monitor platform capacity. Although attempts to accomplish this have been made in the past, these approaches have been excessively complex and not particularly accurate.
An example of such an approach is disclosed in U.S. Pat. No. 4,456,093 (Finley et al.). Here a determination of the load on a work platform mounted on a boom was done by computer by solving simultaneous equations from data produced by pressure transducers attached to the slave hydraulic cylinder and by load cells inside the boom. In other words, the platform load was measured indirectly by reading the force required to keep the platform level and the force carried by the boom. Variables upon which these calculations depended included the elevational angle of the boom, the length the boom was at, and the position of the load on the platform.
Such a system as the one in Finley et al. achieves no more than an indirect approximation of the load weight. This approximation, based on a mathematical formula and requiring multiple sensors for the data needed in that formula, results in limited accuracy at great expense. Moreover, exceptions need to be made by such systems between the monitoring of a static load with that of a live load, i.e. where people or objects are moving or being moved on the platform. Adjustments also need to be made where there is an overhung load, i.e. a portion of the load is overhanging the work basket or material-handling mechanism.
This invention addresses such problems and shortcomings with a mechanism that measures the load directly at the platform in a simple and inexpensive manner.